In recent years, solar batteries have been practically used as a clean and safe energy source. Among various types of solar batteries, heterojunction solar batteries are drawing attention.
FIG. 19 is a cross-sectional view illustrating a general configuration of a typical heterojunction solar battery cell.
As illustrated in FIG. 19, the heterojunction solar battery cell 100 is constructed by stacking, on one side (the sunlight side) of an n-type crystalline silicon substrate 101, an i-type amorphous silicon layer 102, a p-type amorphous silicon layer 103, a first transparent conductive oxide layer 104, and an electrode layer 105 in this order, and stacking, on the other side of the substrate 101, an i-type amorphous silicon layer 106, an n-type amorphous silicon layer 107, a second transparent conductive oxide layer 108, and an electrode layer 109 in this order.
Such a heterojunction solar battery is advantageous in that it provides higher efficiency compared with a single crystalline silicon solar battery, and that it reduces the amount of silicon usage by virtue of the use of amorphous silicon layers.
A further advantage of the heterojunction solar battery is its high power generation efficiency due to its capability of generating power on both sides of an n-type crystalline silicon substrate.
A disadvantage, however, of the heterojunction solar battery is its being more complicated in configuration than a single crystalline silicon solar battery, which results in a larger number of processes for its manufacture and higher cost for a manufacturing apparatus for use therein, as well as higher cost for the solar battery itself.
Further, the presence of transparent conductive oxide layers on both sides of the n-type crystalline silicon substrate in the heterojunction solar battery gives rise to another problem described below.
To enhance the conversion efficiency, it is preferable to form a transparent conductive oxide layer over as large an area as possible on an amorphous silicon layer serving as an underlayer; in other words, over the entire surface of the amorphous silicon layer. If, however, transparent conductive oxide layers are formed over the entire surfaces on both sides of the n-type crystalline silicon substrate by sputtering, materials deposited onto the side surfaces of the substrate due to the sputtering may cause a short-circuit between the transparent conductive oxide layers.
Such a problem may be avoided by forming a transparent conductive oxide layer over the entire surface on one side (for example, the sunlight side) of the n-type crystalline silicon substrate while forming another transparent conductive oxide layer on the other side of the substrate over a region other than the edge region of the substrate by performing sputtering using a mask. This, however, necessitates the use of the mask in forming the layer, thus leading to a cost increase.
The above-described problems are encountered not only in forming transparent conductive oxide layers over both surfaces of a film-formation target substrate for a heterojunction solar battery but also in forming various types of sputtered films over both surfaces of a film-formation target substrate.